CSS Cascading and Inheritance Level 4

1. Introduction

CSS defines a finite set of parameters, called properties, that direct the rendering of a document. Each property has a name (e.g., color, font-size, or border-style), a value space (e.g., , , [ solid | dashed | dotted | … ]), and a defined behavior on the rendering of the document. Properties values are assigned to various parts of the document via property declarations, which assign the property a value (e.g. red, 12pt, dotted) for the associated element or box.

One of the fundamental design principles of CSS is cascading, which allows several style sheets to influence the presentation of a document. When different declarations try to set a value for the same element/property combination, the conflicts must somehow be resolved.

The opposite problem arises when no declarations try to set a value for an element/property combination. In this case, a value is be found by way of inheritance or by looking at the property’s initial value.

The cascading and defaulting process takes a set of declarations as input, and outputs a specified value for each property on each element.

The rules for finding the specified value for all properties on all elements in the document are described in this specification. The rules for finding the specified values in the page context and its margin boxes are described in [css-page-3].

1.1. Module Interactions

This section is normative.

This module replaces and extends the rules for assigning property values, cascading, and inheritance defined in [CSS2] chapter 6.

Other CSS modules may expand the definitions of some of the syntax and features defined here. For example, the Media Queries Level 4 specification, when combined with this module, expands the definition of the value type as used in this specification.

For the purpose of this specification, text nodes are treated as element children of their associated element, and possess the full set of properties; since they cannot be targeted by selectors all of their computed values are assigned by defaulting.

2. Importing Style Sheets: the @import rule

The @import rule allows users to import style rules from other style sheets. If an @import rule refers to a valid stylesheet, user agents must treat the contents of the stylesheet as if they were written in place of the @import rule, with two exceptions:

If a feature (such as the @namespace rule) explicitly defines that it only applies to a particular stylesheet, and not any imported ones, then it doesn’t apply to the imported stylesheet.
If a feature relies on the relative ordering of two or more constructs in a stylesheet (such as the requirement that @namespace rules must not have any other rules other than @import preceding it), it only applies between constructs in the same stylesheet.

For example, declarations in style rules from imported stylesheets interact with the cascade as if they were written literally into the stylesheet at the point of the @import.

Any @import rules must precede all other valid at-rules and style rules in a style sheet (ignoring @charset), or else the @import rule is invalid. The syntax of @import is:

@import [  |  ]
         ;

 = [ supports( [  |  ] ) ]?
                     ?

where the or gives the URL of the style sheet to be imported, and the optional state the import conditions under which it applies.

The following conditional @import rule only loads the style sheet when the UA supports display: flex, and only applies the style sheet on a handheld device with a maximum viewport width of 400px.

@import url("narrow.css") supports(display: flex) handheld and (max-width: 400px);

If a is provided, it must be interpreted as a with the same value.

The following lines are equivalent in meaning and illustrate both @import syntaxes (one with url() and one with a bare string):

@import "mystyle.css";
@import url("mystyle.css");

2.1. Conditional @import Rules

Import conditions allow the import to be media– or feature-support–dependent. In the absence of any import conditions, the import is unconditional. (Specifying all for the has the same effect.) If the import conditions do not match, the rules in the imported stylesheet do not apply, exactly as if the imported stylesheet were wrapped in @media and/or @supports blocks with the given conditions.

The following rules illustrate how @import rules can be made media-dependent:

@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
@import url("narrow.css") handheld and (max-width: 400px);

User agents may therefore avoid fetching a conditional import as long as the import conditions do not match. Additionally, if a blocks the application of the imported style sheet, the UA must not fetch the style sheet (unless it is loaded through some other link) and must return null for the import rule’s CSSImportRule.styleSheet value (even if it is loaded through some other link).

The following rule illustrates how an author can provide rules for modern user agents that support newer web features without impacting network performance on older user agents. In this example, a CSS file which loads COLRv1 fonts is only fetched when COLRv1 is supported:

@import url("COLRv1_fonts.css") supports(font-tech(color-COLRv1));

The import conditions are given by , which is parsed and interpreted as a media query list, and , is parsed and interpreted as a [[supports query]]. If a is given in place of a , it must be interpreted as a (i.e. the extra set of parentheses is implied) and treated as a .

For example, the following two lines are equivalent:

@import "mystyle.css" supports(display: flex);
@import "mystyle.css" supports((display: flex));

The evaluation and full syntax of the import conditions are defined by the Media Queries [MEDIAQ] and CSS Conditional Rules [CSS-CONDITIONAL-3] specifications.

2.2. Processing Stylesheet Imports

When the same style sheet is imported or linked to a document in multiple places, user agents must process (or act as though they do) each link as though the link were to an independent style sheet.

Note: This does not place any requirements on resource fetching, only how the style sheet is reflected in the CSSOM and used in specs such as this one. Assuming appropriate caching, it is perfectly appropriate for a UA to fetch a style sheet only once, even though it’s linked or imported multiple times.

The cascade origin of an imported style sheet is the cascade origin of the style sheet that imported it.

The environment encoding of an imported style sheet is the encoding of the style sheet that imported it. [css-syntax-3]

To fetch an @import, given an @import rule rule:

Let parentStylesheet be rule’s parent CSS style sheet. [CSSOM]
If rule has a , and that condition is not true, return.
Fetch a style resource from rule’s URL, with ruleOrDeclaration rule, destination "style", CORS mode "no-cors", and processResponse being the following steps given response response and byte stream, null or failure byteStream:
1. If byteStream is not a byte stream, return.
2. If parentStylesheet is in quirks mode and response is CORS-same-origin, let content type be "text/css". Otherwise, let content type be the Content Type metadata of response.
3. If content type is not "text/css", return.
4. Let importedStylesheet be the result of parsing byteStream given parsedUrl.
5. Set importedStylesheet’s origin-clean flag to parentStylesheet’s origin-clean flag.
6. If response is not CORS-same-origin, unset importedStylesheet’s origin-clean flag.
7. Set rule’s styleSheet to importedStylesheet.

2.3. Content-Type of CSS Style Sheets

The processing of imported style sheets depends on the actual type of the linked resource:

If the resource does not have Content-Type metadata, the type is treated as text/css.
If the host document is in quirks mode, and the host document’s origin is same origin with the linked resource response’s URL’s origin, the type is treated as text/css.
Otherwise, the type is determined from its Content-Type metadata.

If the linked resource’s type is text/css, it must be interpreted as a CSS style sheet. Otherwise, it must be interpreted as a network error.

3. Shorthand Properties

Some properties are shorthand properties, meaning that they allow authors to specify the values of several properties with a single property. A shorthand property sets all of its longhand sub-properties, exactly as if expanded in place.

When values are omitted from a shorthand form, unless otherwise defined, each “missing” sub-property is assigned its initial value.

This means that a shorthand property declaration always sets all of its sub-properties, even those that are not explicitly set. Carelessly used, this might result in inadvertently resetting some sub-properties. Carefully used, a shorthand can guarantee a “blank slate” by resetting sub-properties inadvertently cascaded from other sources.

For example, writing background: green rather than background-color: green ensures that the background color overrides any earlier declarations that might have set the background to an image with background-image.

For example, the CSS Level 1 font property is a shorthand property for setting font-style, font-variant, font-weight, font-size, line-height, and font-family all at once. The multiple declarations of this example:

h1 {
  font-weight: bold;
  font-size: 12pt;
  line-height: 14pt;
  font-family: Helvetica;
  font-variant: normal;
  font-style: normal;
}

can therefore be rewritten as

h1 { font: bold 12pt/14pt Helvetica }

As more font sub-properties are introduced into CSS, the shorthand declaration resets those to their initial values as well.

In some cases, a shorthand might have different syntax or special keywords that don’t directly correspond to values of its sub-properties. (In such cases, the shorthand will explicitly define the expansion of its values.)

In other cases, a property might be a reset-only sub-property of the shorthand: Like other sub-properties, it is reset to its initial value by the shorthand when unspecified, but the shorthand might not include syntax to set the sub-property to any of its other values. For example, the border shorthand resets border-image to its initial value of none, but has no syntax to set it to anything else. [css-backgrounds-3]

If a shorthand is specified as one of the CSS-wide keywords [css-values-3], it sets all of its sub-properties to that keyword, including any that are reset-only sub-properties. (Note that these keywords cannot be combined with other values in a single declaration, not even in a shorthand.)

Declaring a shorthand property to be !important is equivalent to declaring all of its sub-properties to be !important.

3.1. Property Aliasing

Properties sometimes change names after being supported for a while, such as vendor-prefixed properties being standardized. The original name still needs to be supported for compatibility reasons, but the new name is preferred. To accomplish this, CSS defines two different ways of “aliasing” old syntax to new syntax.

legacy name aliases: When the old property’s value syntax is identical to that of the new property, the two names are aliased with an operation on par with case-mapping: at parse time, the old property is converted into the new property. This conversion also applies in the CSSOM, both for string arguments and property accessors: requests for the old property name transparently transfer to the new property name instead.
For example, if old-name is a legacy name alias for new-name, getComputedStyle(el).oldName will return the computed style of the newName property, and el.style.setPropertyValue("old-name", "value") will set the new-name property to "value".
legacy shorthands: When the old property has a distinct syntax from the new property, the two names are aliased using the shorthand mechanism. These shorthands are defined to be legacy shorthands, and their use is deprecated. They otherwise behave exactly as regular shorthands, except that the CSSOM will not use them when serializing declarations. [CSSOM]
For example, the page-break-* properties are legacy shorthands for the break-* properties (see CSS Fragmentation 3 § 3.4 Page Break Aliases: the page-break-before, page-break-after, and page-break-inside properties).
Setting page-break-before: always expands to break-before: page at parse time, like other shorthands do. Similarly, if break-before: page is set, calling getComputedStyle(el).pageBreakBefore will return "always". However, when serializing a style block (see CSSOM § 6.7.2 Serializing CSS Values), the page-break-before property will never be chosen as the shorthand to serialize to, regardless of whether it or break-before was specified; instead, break-before will always be chosen.

3.2. Resetting All Properties: the all property

Name:	all
Value:	initial \| inherit \| unset \| revert
Initial:	see individual properties
Applies to:	see individual properties
Inherited:	see individual properties
Percentages:	see individual properties
Computed value:	see individual properties
Animation type:	see individual properties
Canonical order:	per grammar

The all property is a shorthand that resets all CSS properties except direction and unicode-bidi. It only accepts the CSS-wide keywords. It does not reset custom properties [css-variables-1].

Note: The excepted CSS properties direction and unicode-bidi are actually markup-level features, and should not be set in the author’s style sheet. (They exist as CSS properties only to style document languages not supported by the UA.) Authors should use the appropriate markup, such as HTML’s dir attribute, instead. [css-writing-modes-3]

For example, if an author specifies all: initial on an element, it will block all inheritance and reset all properties, as if no rules appeared in the author, user, or user-agent levels of the cascade.

This can be useful for the root element of a "widget" included in a page, which does not wish to inherit the styles of the outer page. Note, however, that any "default" style applied to that element (such as, e.g. display: block from the UA style sheet on block elements such as

) will also be blown away.

4. Value Processing

Once a user agent has parsed a document and constructed a document tree, it must assign, to every element in the flat tree, and correspondingly to every box in the formatting structure, a value to every property that applies to the target media type.

The final value of a CSS property for a given element or box is the result of a multi-step calculation:

First, all the declared values applied to an element are collected, for each property on each element. There may be zero or many declared values applied to the element.
Cascading yields the cascaded value. There is at most one cascaded value per property per element.
Defaulting yields the specified value. Every element has exactly one specified value per property.
Resolving value dependencies yields the computed value. Every element has exactly one computed value per property.
Formatting the document yields the used value. An element only has a used value for a given property if that property applies to the element.
Finally, the used value is transformed to the actual value based on constraints of the display environment. As with the used value, there may or may not be an actual value for a given property on an element.

Elements that are not connected or are not part of the document’s flattened element tree do not participate in CSS value processing, and do not have declared, cascaded, specified, computed, used, or actual values, even if they potentially have style declarations assigned to them (for example, by a style attribute).

4.1. Declared Values

Each property declaration applied to an element contributes a declared value for that property associated with the element. See Filtering Declarations for details.

These values are then processed by the cascade to choose a single “winning value”.

4.1.1. Value Aliasing

Some property values have legacy value aliases: at parse time, the legacy syntax is converted into the new syntax, resulting in a declared value different from the parsed input. These aliases are typically used for handling legacy compatibility requirements, such as converting vendor-prefixed values to their standard equivalents.

4.2. Cascaded Values

The cascaded value represents the result of the cascade: it is the declared value that wins the cascade (is sorted first in the output of the cascade). If the output of the cascade is an empty list, there is no cascaded value.

4.3. Specified Values

The specified value is the value of a given property that the style sheet authors intended for that element. It is the result of putting the cascaded value through the defaulting processes, guaranteeing that a specified value exists for every property on every element.

In many cases, the specified value is the cascaded value. However, if there is no cascaded value at all, the specified value is defaulted. The CSS-wide keywords are handled specially when they are the cascaded value of a property, setting the specified value as required by that keyword, see § 7.3 Explicit Defaulting.

4.4. Computed Values

The computed value is the result of resolving the specified value as defined in the “Computed Value” line of the property definition table, generally absolutizing it in preparation for inheritance.

Note: The computed value is the value that is transferred from parent to child during inheritance. For historical reasons, it is not necessarily the value returned by the getComputedStyle() function, which sometimes returns used values. [CSSOM] Furthermore, the computed value is an abstract data representation: their definitions reflect that data representation, not how that data is serialized. For example, serialization rules often allow omitting certain values which are implied during parsing; but those values are nonetheless part of the computed value.

A specified value can be either absolute (i.e., not relative to another value, as in red or 2mm) or relative (i.e., relative to another value, as in auto, 2em). Computing a relative value generally absolutizes it:

values with relative units (em, ex, vh, vw) must be made absolute by multiplying with the appropriate reference size
certain keywords (e.g., smaller, bolder) must be replaced according to their definitions
percentages on some properties must be multiplied by a reference value (defined by the property)
valid relative URLs must be resolved to become absolute.

See examples (f), (g) and (h) in the table below.

Note: In general, the computed value resolves the specified value as far as possible without laying out the document or performing other expensive or hard-to-parallelize operations, such as resolving network requests or retrieving values other than from the element and its parent.

The computed value exists even when the property does not apply. However, some properties may change how they determine the computed value based on whether the property applies to the element.

4.5. Used Values

The used value is the result of taking the computed value and completing any remaining calculations to make it the absolute theoretical value used in the formatting of the document.

For example, a declaration of width: auto can’t be resolved into a length without knowing the layout of the element’s ancestors, so the computed value is auto, while the used value is an absolute length, such as 100px. [CSS2]

As another example, a

might have a computed break-before value of auto, but acquire a used break-before value of page by propagation from its first child. [css-break-3]

If a property does not apply to this element or box type then it has no used value for that property.

For example, the flex property has no used value on elements that aren’t flex items.

4.5.1. Applicable Properties

If a property does not apply to an element or box type—as noted in its “Applies to” line—this means it does not directly take effect on that type of box or element.

Note: A property that does not apply can still have indirect formatting effects if its computed value affects the computation of other properties that do apply; and of course its computed value, which always exists, can still inherit to descendants and take effect on them.

Even though writing-mode and text-orientation do not apply to table rows (they do not affect how the table row or its children are laid out), setting them on such boxes will still affect the calculation of font relative units such as ch, and thus possibly any property that takes a .

Setting text-transform on an HTML p element (which is display: block by default) will have an effect, even though text-transform only applies to inline boxes, because the property inherits into the paragraph’s anonymous root inline box and applies to the text it contains.

Note: A property defined to apply to “all elements” applies to all elements and display types, but not necessarily to all pseudo-element types, since pseudo-elements often have their own specific rendering models or other restrictions. The ::before and ::after pseudo-elements, however, are defined to generate boxes almost exactly like normal elements and are therefore defined accept all properties that apply to “all elements”. See [CSS-PSEUDO-4] for more information about pseudo-elements.

4.6. Actual Values

A used value is in principle ready to be used, but a user agent may not be able to make use of the value in a given environment. For example, a user agent may only be able to render borders with integer pixel widths and may therefore have to approximate the used width. Also, the font size of an element may need adjustment based on the availability of fonts or the value of the font-size-adjust property. The actual value is the used value after any such adjustments have been made.

Note: By probing the actual values of elements, much can be learned about how the document is laid out. However, not all information is recorded in the actual values. For example, the actual value of the page-break-after property does not reflect whether there is a page break or not after the element. Similarly, the actual value of orphans does not reflect how many orphan lines there is in a certain element. See examples (j) and (k) in the table below.

4.7. Examples

Examples of CSS Value Computation
	Property	Winning declaration	Cascaded value	Specified value	Computed value	Used value	Actual value
(a)	text-align	`text-align: left`	left	left	left	left	left
(b)	border-top-width, border-right-width, border-bottom-width, border-left-width	`border-width: inherit`	inherit	4.2px	4.2px	4.2px	4px
(c)	width	(none)	(none)	auto (initial value)	auto	120px	120px
(d)	list-style-position	`list-style-position: inherit`	inherit	inside	inside	inside	inside
(e)	list-style-position	`list-style-position: initial`	initial	outside (initial value)	outside	outside	outside
(f)	font-size	`font-size: 1.2em`	1.2em	1.2em	14.1px	14.1px	14px
(g)	width	`width: 80%`	80%	80%	80%	354.2px	354px
(h)	width	`width: auto`	auto	auto	auto	134px	134px
(i)	height	`height: auto`	auto	auto	auto	176px	176px
(j)	page-break-after	(none)	(none)	auto (initial value)	auto	auto	auto
(k)	orphans	`orphans: 3`	3	3	3	3	3

5. Filtering

In order to find the declared values, implementations must first identify all declarations that apply to each element. A declaration applies to an element if:

It belongs to a style sheet that currently applies to this document.
It is not qualified by a conditional rule [CSS-CONDITIONAL-3] with a false condition.
It belongs to a style rule whose selector matches the element. [SELECT] (Taking scoping into account, if necessary.)
It is syntactically valid: the declaration’s property is a known property name, and the declaration’s value matches the syntax for that property.

The values of the declarations that apply form, for each property on each element, a list of declared values. The next section, the cascade, prioritizes these lists.

6. Cascading

The cascade takes an unordered list of declared values for a given property on a given element, sorts them by their declaration’s precedence as determined below, and outputs a single cascaded value.

6.1. Cascade Sorting Order

The cascade sorts declarations according to the following criteria, in descending order of precedence:

Origin and Importance

The origin of a declaration is based on where it comes from and its importance is whether or not it is declared with !important (see below). The precedence of the various origins is, in descending order:

Transition declarations [css-transitions-1]
Important user agent declarations
Important user declarations
Important author declarations
Animation declarations [css-animations-1]
Normal author declarations
Normal user declarations
Normal user agent declarations

Declarations from origins earlier in this list win over declarations from later origins.

Context

A document language can provide for blending declarations sourced from different encapsulation contexts, such as the nested tree contexts of shadow trees in the [DOM].

When comparing two declarations that are sourced from different encapsulation contexts, then for normal rules the declaration from the outer context wins, and for important rules the declaration from the inner context wins. For this purpose, [DOM] tree contexts are considered to be nested in shadow-including tree order.

Note: This effectively means that normal declarations belonging to an encapsulation context can set defaults that are easily overridden by the outer context, while important declarations belonging to an encapsulation context can enforce requirements that cannot be overridden by the outer context.

Specificity

The Selectors module [SELECT] describes how to compute the specificity of a selector. Each declaration has the same specificity as the style rule it appears in. For the purpose of this step, declarations that do not belong to a style rule (such as the contents of a style attribute) are considered to have a specificity higher than any selector. The declaration with the highest specificity wins.

Order of Appearance

The last declaration in document order wins. For this purpose:

Style sheets are ordered in [[final CSS style sheets]] order.
Declarations from imported style sheets are ordered as if their style sheets were substituted in place of the @import rule.
Declarations from style sheets independently linked by the originating document are treated as if they were concatenated in linking order, as determined by the host document language.
Declarations from style attributes are ordered according to the document order of the element the style attribute appears on, and are all placed after any style sheets. [CSSSTYLEATTR]

The output of the cascade is a (potentially empty) sorted list of declared values for each property on each element.

6.2. Cascading Origins

Each style rule has a cascade origin, which determines where it enters the cascade. CSS defines three core origins:

Author Origin: The author specifies style sheets for a source document according to the conventions of the document language. For instance, in HTML, style sheets may be included in the document or linked externally.
User Origin: The user may be able to specify style information for a particular document. For example, the user may specify a file that contains a style sheet or the user agent may provide an interface that generates a user style sheet (or behaves as if it did).
User-Agent Origin: Conforming user agents must apply a default style sheet (or behave as if they did). A user agent’s default style sheet should present the elements of the document language in ways that satisfy general presentation expectations for the document language (e.g., for visual browsers, the EM element in HTML is presented using an italic font). See e.g. the HTML user agent style sheet. [HTML]

Extensions to CSS define the following additional origins:

Animation Origin: CSS Animations [css-animations-1] generate “virtual” rules representing their effects when running.
Transition Origin: Like CSS Animations, CSS Transitions [css-transitions-1] generate “virtual” rules representing their effects when running.

6.3. Important Declarations: the !important annotation

CSS attempts to create a balance of power between author and user style sheets. By default, rules in an author’s style sheet override those in a user’s style sheet, which override those in the user-agent’s default style sheet. To balance this, a declaration can be marked important, which increases its weight in the cascade and inverts the order of precedence.

A declaration is important if it has a !important annotation as defined by [css-syntax-3], i.e. if the last two (non-whitespace, non-comment) tokens in its value are the delimiter token ! followed by the identifier token important. All other declarations are normal (non-important).

[hidden] { display: none !important; }

An important declaration takes precedence over a normal declaration. Author and user style sheets may contain important declarations, with user-origin important declarations overriding author-origin important declarations. This CSS feature improves accessibility of documents by giving users with special requirements (large fonts, color combinations, etc.) control over presentation.

Important declarations from all origins take precedence over animations. This allows authors to override animated values in important cases. (Animated values normally override all other rules.) [css-animations-1]

User-agent style sheets may also contain important declarations. These override all author and user declarations.

The first rule in the user’s style sheet in the following example contains an !important declaration, which overrides the corresponding declaration in the author’s style sheet. The declaration in the second rule will also win due to being marked !important. However, the third declaration in the user’s style sheet is not !important and will therefore lose to the second rule in the author’s style sheet (which happens to set style on a shorthand property). Also, the third author rule will lose to the second author rule since the second declaration is !important. This shows that !important declarations have a function also within author style sheets.

/* From the user’s style sheet */
p { text-indent: 1em !important }
p { font-style: italic !important }
p { font-size: 18pt }

/* From the author’s style sheet */
p { text-indent: 1.5em !important }
p { font: normal 12pt sans-serif !important }
p { font-size: 24pt }

Property	Winning value
text-indent	1em
font-style	italic
font-size	12pt
font-family	sans-serif

6.4. Precedence of Non-CSS Presentational Hints

The UA may choose to honor presentational hints in a source document’s markup, for example the bgcolor attribute or s element in [HTML]. All document language-based styling must be translated to corresponding CSS rules and enter the cascade as rules in either the UA-origin or a special-purpose author presentational hint origin between the regular user origin and the author origin. For the purpose of cascading this author presentational hint origin is treated as an independent origin, but for the purpose of the revert keyword it is considered part of the author origin.

A document language may define whether such a presentational hint enters the cascade as UA-origin or author-origin; if so, the UA must behave accordingly. For example, [SVG11] maps its presentation attributes into the author origin.

Note: Presentational hints entering the cascade as UA-origin rules can be overridden by author-origin or user-origin styles. Presentational hints entering the cascade as author presentational hint origin rules can be overridden by author-origin styles, but not by non-important user-origin styles. Host languages should choose the appropriate origin for presentational hints with these considerations in mind.

7. Defaulting

When the cascade does not result in a value, the specified value must be found some other way. Inherited properties draw their defaults from their parent element through inheritance; all other properties take their initial value. Authors can explicitly request inheritance or initialization via the inherit and initial keywords.

7.1. Initial Values

Each property has an initial value, defined in the property’s definition table. If the property is not an inherited property, and the cascade does not result in a value, then the specified value of the property is its initial value.

7.2. Inheritance

Inheritance propagates property values from parent elements to their children. The inherited value of a property on an element is the computed value of the property on the element’s parent element. For the root element, which has no parent element, the inherited value is the initial value of the property.

For a [DOM] tree with shadows, inheritance operates on the flattened element tree. This means that slotted elements inherit from the slot they’re assigned to, rather than directly from their light tree parent. Pseudo-elements inherit according to the fictional tag sequence described for each pseudo-element. [CSS-PSEUDO-4]

Some properties are inherited properties, as defined in their property definition table. This means that, unless the cascade results in a value, the value will be determined by inheritance.

A property can also be explicitly inherited. See the inherit keyword.

Note: Inheritance follows the document tree and is not intercepted by anonymous boxes, or otherwise affected by manipulations of the box tree.

7.3. Explicit Defaulting

Several CSS-wide property values are defined below; declaring a property to have these values explicitly specifies a particular defaulting behavior. As specified in CSS Values and Units [css-values-3], all CSS properties can accept these values.

The keyword revert is a cascade-dependent keyword; some contexts may restrict its use while allowing the other CSS-wide keywords.

7.3.1. Resetting a Property: the initial keyword

The initial CSS-wide keyword represents the value defined as the property’s initial value.

If the cascaded value of a property is the initial keyword, the property’s specified value is its initial value.

7.3.2. Explicit Inheritance: the inherit keyword

The inherit CSS-wide keyword represents the property’s computed value on the parent element.

If the cascaded value of a property is the inherit keyword, the property’s specified and computed values are the inherited value.

7.3.3. Erasing All Declarations: the unset keyword

The unset CSS-wide keyword acts as either inherit or initial, depending on whether the property is inherited or not.

If the cascaded value of a property is the unset keyword, then if it is an inherited property, this is treated as inherit, and if it is not, this is treated as initial. This keyword effectively erases all declared values occurring earlier in the cascade, correctly inheriting or not as appropriate for the property (or all longhands of a shorthand).

7.3.4. Rolling Back Cascade Origins: the revert keyword

The revert CSS-wide keyword rolls back the cascade to the cascaded value of the earlier origin.

If the cascaded value of a property is the revert keyword, the behavior depends on the cascade origin to which the declaration belongs:

user-agent origin: Equivalent to unset.
user origin: Rolls back the cascaded value to the user-agent level, so that the specified value is calculated as if no author-origin or user-origin rules were specified for this property on this element.
author origin: Rolls back the cascaded value to the user level, so that the specified value is calculated as if no author-origin rules were specified for this property on this element. For the purpose of revert, this origin includes the Animation origin.

Note: Effectively the revert keyword is substituted with the value from the earlier cascade origin. Thus, reverting a shorthand property reverts all its longhands; reverting any property alias of a property reverts all of them; reverting one of the paired properties in a logical property group also reverts the other one; etc.

8. Changes

8.1. Changes since the 13 Jan 2022 Candidate Recommendation Snapshot

Non-trivial changes since the 13 Jan 2022 Candidate Recommendation Snapshot:

Clarify that all “aliases” of a property are reverted by revert.
Clarify that style sheets are ordered in [[final CSS style sheets]] order.

8.2. Changes since the 15 Oct 2021 Working Draft

Non-trivial changes since the 15 October 2021 Working Draft:

Updated @import grammar for media queries and supports conditions
Allowed functional notation parse-time aliases
Defined fetching an @import, in terms of Fetch
Added § 4.1.1 Value Aliasing section. (Issue 6193)

8.3. Changes Since the 28 August 2018 Candidate Recommendation

Non-trivial changes since the 19 March 2021 Working Draft include:

Defined author presentational hint origin to handle author-origin presentational hints, instead of relying on zero-specificity and source order, to correctly define their interaction with the encapsulation context aspect of the cascade. (Issue 66749)

Non-trivial changes since the 18 August 2020 Working Draft include:

Removed possibility of legacy name aliases to map subsets of the value space, since they are simple name aliases. (Issue 4839)
Gave concept of applies to its own section and add some notes about implications. (Issues 1861 and 5565)
Defined the term property. (Issue 5633)
Defined value processing of elements that are not part of the tree. (Issue 1964 and 1548)

Elements that are not connected or are not part of the document’s flattened element tree do not participate in CSS value processing, and do not have declared, cascaded, specified, computed, used, or actual values, even if they potentially have style declarations assigned to them (for example, by a style attribute).
Clarify origin comparison for quirks mode Content-Type assumptions in § 2.3 Content-Type of CSS Style Sheets. (Issue 4838)

Non-trivial changes since the 28 August 2018 Candidate Recommendation include:

Added context to the cascade sort criteria to accommodate Shadow DOM. [DOM] (Issue 5372)
Defined that, in consideration of shadow trees, inheritance operates over the flattened element tree.
Removed scoping from the cascade sort criteria, because it has not been implemented.

8.4. Changes Since the 14 January 2016 Candidate Recommendation

Non-trivial changes since the 14 January 2016 Working Draft include:

Precisely defined the types of aliasing that CSS uses. (Issue 866) See § 3.1 Property Aliasing.
Clarified that revert only affects the cascaded value, not the inherited value.

user origin
Rolls back the ~~cascade~~ cascaded value to the user-agent level, so that the specified value is calculated as if no author-level or user-level rules were specified for this property on this element .
author origin
Rolls back the ~~cascade~~ cascaded value to the user level, so that the specified value is calculated as if no author-level rules were specified for this property on this element .
Clarified that custom properties are not reset by the all shorthand. (2518)

The all property is a shorthand that resets all CSS properties except direction and unicode-bidi. … It does not reset custom properties [css-variables-1].
Defined more precisely that imported stylesheets are interpreted separately from the importing stylesheet, in terms of ordering of rules, etc.
If an @import rule refers to a valid stylesheet, user agents must treat the contents of the stylesheet as if they were written in place of the @import rule , with two exceptions:
- If a feature (such as the @namespace rule) explicitly defines that it only applies to a particular stylesheet, and not any imported ones, then it doesn’t apply to the imported stylesheet.
- If a feature relies on the relative ordering of two or more constructs in a stylesheet (such as the requirement that @charset must not have any other content preceding it), it only applies between constructs in the same stylesheet.
Specified that text nodes are considered children of their parent element, and receive styles via defaulting, as their properties are now observable distinct from their inline parent’s via display: contents [css-display-3].

For the purpose of this specification, text nodes are treated as element children of their associated element, and possess the full set of properties; since they cannot be targeted by selectors all of their computed values are assigned by defaulting.
Removed any mention of the obsolete “override” origin, originally defined by DOM Level 2 Style and later abandoned. (Issue 1385)

A Disposition of Comments is available.

8.5. Changes Since the 21 April 2015 Working Draft

Changes since the 21 April 2015 Working Draft include:

Renamed default keyword to revert.
Allowed dropping duplicate parentheses in supports() syntax when it only contains one declaration.

8.6. Additions Since Level 3

The following features have been added since Level 3:

Introduced revert keyword, for rolling back the cascade.
Introduced supports() syntax for supports-conditional @import rules.
Added encapsulation context to the cascade sort criteria to accommodate Shadow DOM. [DOM]
Defined the property two aliasing mechanisms CSS uses to support legacy syntaxes. See § 3.1 Property Aliasing.

8.7. Additions Since Level 2

The following features have been added since Level 2:

The all shorthand
The initial keyword
The unset keyword
Incorporation of animations and transitions into the cascade.

Acknowledgments

David Baron, Tantek Çelik, Noam Rosenthal, Simon Sapin, and Boris Zbarsky contributed to this specification.

Privacy and Security Considerations

The cascade process does not distinguish between same-origin and cross-origin stylesheets, enabling the content of cross-origin stylesheets to be inferred from the computed styles they apply to a document.
User preferences and UA defaults expressed via application of style rules are exposed by the cascade process, and can be inferred from the computed styles they apply to a document.
The @import rule does not apply the CORS protocol to loading cross-origin stylesheets, instead allowing them to be freely imported and applied.
The @import rule assumes that resources without Content-Type metadata (or any same-origin file if the host document is in quirks mode) are text/css, potentially allowing arbitrary files to be imported into the page and interpreted as CSS, potentially allowing sensitive data to be inferred from the computed styles they apply to a document.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with , like this: UAs MUST provide an accessible alternative.